With the evolving of semiconductor technologies, semiconductor chips/dies are becoming increasingly smaller. In the meantime, more functions need to be integrated into the semiconductor dies. Accordingly, the semiconductor dies need to have increasingly greater numbers of I/O connections packed into smaller areas, and the density of the I/O connections rises quickly over time. As a result, the packaging of the semiconductor dies becomes more difficult, which adversely affects the yield of the packaging.
The increase in the density of the I/O connections results in the reduction in the pitch of the IO connections. As a result, the closely located solder regions, which are used to form the IO connections in the bonding processes, are more likely to bridge with each other.
One of the methods for packaging integrated circuits is die stacking, which is also sometimes referred to as three-dimensional (3D) die stacking. 3D Die stacking structures have reduced footages than the packages in which dies are disposed on the same plane. For example, in a conventional package, a first die is bonded to a second die through-flip chip bonding, wherein the second die has a larger area than the first die. The second die includes additional bond pads on the same side as the first die, which additional bond pads are not used for bonding to the first die. A third die is then bonded to the second die through flip-chip bonding, wherein the third die is bonded to the additional bond pads on the second die.
The third die has an area larger than the areas of both the first die and the second die. The third die, in addition to the bond pads that are used to bond to the second die, also includes extra bond pads that are on the same side (of the third die) as the first die and the second die. The third die may then be bonded to a package substrate through the extra bond pads.
In the above-discussed package, the first die and the second die have a first distance. The second die and the third die have a second distance greater than the first distance. The third die and the package substrate have a third distance greater than the second distance. Accordingly, the solder balls that are used to bond the third die to the second die are larger than the solder balls that are used to bond the first die to the second die. The solder balls that are used to bond the package substrate to the third die are larger than the solder balls that are used to bond the third die to the second die. The increase in the sizes of the solder balls causes the increase in the required size of the third die and the package substrate. In addition, the increase in the sizes of the solder balls also results in the increase in the pitches of the solder balls.